Synthetic aperture (SA) imaging is a well-known imaging technology that can be used to increase resolution beyond the diffraction limit of a physical aperture of an imaging system. In SA imaging systems, a large “virtual” aperture is synthesized along a path by coherently summing the amplitude and phase information of return echoes from a plurality of electromagnetic signals sequentially transmitted by a relatively small physical aperture provided on a platform moving along the path. Typical implementations of SA imaging systems include a transmitter-receiver unit mounted on an airborne, spaceborne, or terrestrial platform (e.g., an aircraft, a satellite, a ground vehicle, a watercraft, and the like) traveling along a path over a target region to be imaged. The transmitter-receiver unit directs a plurality of electromagnetic signals onto the target region and collects a series of phase-coherent return echoes corresponding to the electromagnetic signals reflected by the target region. The return echoes can be recorded, and then coherently combined using signal processing techniques to reconstruct a high-resolution image of the target region. Typical implementations of SA imaging systems achieve two-dimensional imaging by using phase history reconstruction along the path (also referred to as the “azimuth” or “along-track” direction) and ranging with chirped signals at an angle (e.g., perpendicularly in zero-squint mode) to the path (also referred to as the “range” or “beam” direction).
SA imaging technology was initially developed and has been successfully employed at radio frequencies, where it is referred to as “synthetic aperture radar” (SAR) imaging. Conventional SAR systems typically operate in the centimeter (cm) wavelength range and produce images with azimuth resolutions of the order of a meter for spaceborne applications and of the order of a decimeter for airborne applications. As resolution is generally inversely proportional to the wavelength used for imaging, there has been a growing interest to extend SAR technology to shorter wavelengths. In particular, an emerging technology referred to as “synthetic aperture lidar” (SAL) imaging is currently being developed in order to apply SAR technology to the visible and near-infrared portions of the electromagnetic spectrum, with most reported experimental studies of SAL dating from the last decade. It is envisioned that SAL could produce images with azimuth resolutions of centimeters or less, and also provide information complementary to that provided by SAR systems.
In addition to its promising potential in terms of resolution, the development of SAL imaging also poses a number of challenges, among which is the measurement and correction of phase errors. As SA imaging relies on maintaining phase coherence between the return echoes collected over the length of the virtual aperture, any uncompensated fluctuations in the length of the optical path between the SA imaging system and the target region to be imaged can affect the phase of the return echoes and, in turn, lead to phase errors that can degrade the image reconstruction process. In particular, phase errors can result in images that are not uniformly focused across the target region. Typical sources of uncompensated optical-path-length fluctuations include, for example, unintended deviations in the platform motion and refractive-index inhomogeneities in the atmosphere. As obtaining high-quality SA images generally involves keeping phase errors to within a fraction of the imaging wavelength, which becomes increasingly difficult as the imaging wavelength decreases, phase errors are expected to be more important in SAL than in SAR.
One phase error correction method used in SAR systems employs global positioning system (GPS) data with an inertial navigation system (INS) to provide real-time compensation of undesired platform motions, in combination with autofocus techniques such as the phase gradient autofocus (PGA) algorithm. The PGA algorithm is a state-of-the-art technique for phase error correction that exploits the redundancy of phase error information among range bins by selecting and synthesizing the strongest scatterers (which may be in situ corner-cube retroreflectors) for each range bin. However, implementing an integrated INS/GPS system is generally complex and may not be sufficiently accurate for SAL requirements. Also, the PGA algorithm tends to be less efficient for large phase errors, and may therefore not be suitable for being used alone in SAL, due to the high level of blurring generally observed in uncorrected SAL images.
Accordingly, various challenges still exist in the field of phase error correction in SA imaging applications, particularly in SAL applications.